High strength steel

ABSTRACT

A steel composition that includes: about 0.25-0.37% by weight Carbon; about 1.20-1.55% by weight Manganese; about 0.1-0.15% by weight Vanadium; about 0.20-0.40% by weight Nickel; about 0.20-0.50% by weight Silicon; about 0.30-0.45% by weight Copper; about 0.017-0.025% by weight Nitrogen; and Iron as the main constituent.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No. 11/092,434, filed Mar. 29, 2005, which claims priority to U.S. provisional application No. 60/557,367, filed Mar. 29, 2004, all of which are relied on and incorporated by reference.

BACKGROUND

Articles such as anchor bolts are used in the utility industry to secure transmission poles to concrete bases. Such articles require a strong material that exhibits good low temperature impact strength. One suitable material for such articles is steel having a minimum Charpy v-notch impact strength at 20 F of 15 Ft-lb and minimum yield strength of 75,000 psi. Such steels are typically manufactured according to a process that involves normalizing the steel in a furnace at high temperatures, followed by a separate, high temperature tempering treatment, to ensure production of steels that consistently have the required mechanical properties.

DETAILED DESCRIPTION OF THE INVENTION

Advantageous steel compositions are described that include iron as the main constituent and the following additional elements:

(a) about 0.25-0.37% (preferably about 0.30-0.34%, more preferably about 0.30-0.32%) by weight carbon;

(b) about 1.20-1.55% (preferably about 1.25-1.50%, more preferably about 1.35-1.45%) by weight manganese;

(c) about 0.1-0.15% (preferably about 0.11-0.14%) by weight vanadium;

(d) about 0.20-0.40% by weight nickel;

(e) about 0.20-0.50% by weight silicon;

(f) about 0.30-0.45% by weight copper; and

(g) about 0.017-0.025% (preferably about 0.018-0.022%, more preferably about 0.019-0.021%) by weight nitrogen. The composition may also include one or more of the following elements:

(h) up to about 0.30% (preferably up to about 0.25%, more preferably up to about 0.20%) by weight chromium;

(i) up to about 0.035% (preferably up to about 0.025%, more preferably up to about 0.020%) by weight phosphorus;

(j) up to about 0.04% by weight sulfur (preferably up to about 0.02%);

(k) up to about 0.06% by weight tin; and/or

(l) up to about 0.06% (preferably up to about 0.04%) by weight molybdenum.

In some embodiments, Ti, Nb and Al can be present individually or in combination in amounts of up to 0.025% by weight Ti, up to 0.025% by weight Nb and up to 0.04% Al.

Other elements may also be present in the steel in low percentages.

To prepare steel having good mechanical properties (e.g., steels having good low temperature impact strength coupled with high yield and tensile strength such as 75S steel), the above-described steel composition is charged to a furnace, where it is normalized by heating the composition at a furnace temperature between about 1500° F. and about 1650° F. Optionally, the composition may be further treated by tempering the composition by heating at a furnace temperature between about 850° F. and about 1000° F. However, one advantage of the composition is that steels having mechanical properties sufficient for applications such as anchor bolts may be consistently produced without the separate tempering step. The ability to eliminate the tempering step, in turn, reduces the overall cost of producing the steel product.

Minor amounts of other elements may also be present in the steel.

The individual effect of the various elements in an alloy is obscured by the presence of other elements. Together, the combination of elements in the steel alloy provides the desired properties. Although the individual effect of the elements cannot be easily isolated from the combined effect of the alloy, it is generally recognized that certain elements will have certain effects. The various elements and their generally recognized effects can be described as follows.

Iron, Fe, is the main element in steel.

Carbon, C, is a principal element responsible for hardness in steel and a wide range of other properties including strength, ductility, impact strength, etc. Generally, carbon increases tensile strength and decreases ductility.

Manganese, Mn, as an element in steel generally increases hardenability, toughness, and tensile strength of the steel, though it may decrease ductility. Manganese helps in stabilizing steel microstructures and helps prevent degradation of iron carbide structures to iron and graphite. Manganese can also help offset negative effects of other elements, and can assist in reducing brittleness and possible tearing of the steel.

Silicon, Si, acts as a deoxidizer of steel. Silicon can improve tensile strength, but reduces machinability and can promote graphitization.

Copper, Cu, can cause tearing and poor surface quality of the steel. Cooper can stiffen the steel, but decreases ductility. Cooper also imparts corrosion resistance to the steel.

Nickel, Ni, improves hardenability and stiffens steel, but it decreases ductility. Nickel acts to reduce distortion in heat-treating and enables milder quenching. Nickel also improves fatigue properties, toughness, corrosion resistance, and also improves the surface quality of steel.

Chromium, Cr, improves wear resistance and improves the resistance to softening during heat-treating. Chromium also stiffens steel and reduces ductility and improves hardenability, but can increase the brittleness of steel.

Molybdenum, Mo, can greatly increase hardenability. It also increases stiffness and decreases ductility. Molybdenum can improve control of heat treatment by inhibiting formation of certain steel microstructures. It can also increase corrosion resistance, toughness, and fatigue properties. Molybdenum can also be particularly expensive.

Vanadium, V, can help control the steel grain size and reduces the growth of austenite structures. Vanadium also improves abrasion resistance, and improves yield strength, toughness, and hardness. It also can be particularly expensive.

Nitrogen, N, can increase the strength of steel and improve weldability. It also increases brittleness and can lead to increased porosity of the steel.

Phosphorus, P, can improve hardenability and corrosion resistance. It also can improve machinability of the steel. However, it decreases ductility and impact strength, sometimes significantly. Control of phosphorus content can also affect the required heat time in steel preparation.

Sulfur, S, is used to improve machinability. Generally, it decreases impact strength, ductility, and weldability. It also can decrease surface quality and may lead to tearing.

Tin, Sn, is generally used to coat steels. As an alloy element, Tin decreases surface quality and may lead to tearing. It also increases brittleness of the steel.

Titanium, Ti, and Niobium, Nb, provide grain refinement, precipitation strengthening and sulfide shape control by forming a number of compounds like nitrides and carbides. Titanium and Aluminum, Al, act as strong deoxiders of steel as well. This group of elements improves yield strength and toughness.

To produce steel having useful mechanical properties suitable for applications such as anchor bolts, the composition is charged to a furnace, where it is normalized by heating the composition at a furnace temperature between about 1500° F. and about 1650° F. The composition may be in the form of, for example, bars, ingots, plates, sheets, or the like. The composition, if desired, may be further treated by tempering the composition by heating at a furnace temperature between about 850° F. and about 1000° F. However, the tempering is not required and is preferably eliminated, thereby lowering overall production costs.

The normalization step may be performed by charging the composition at an initial furnace temperature at about 1600° F., and then lowering the furnace temperature to a furnace temperature at about 1500° F. once the composition temperature approaches 1500° F. In one approach, the composition is held at the initial furnace temperature for about 15 to 30 minutes, and then held at the second furnace temperature for about 30 to 45 minutes. The first part of the process can be referred to as the “thermal head,” while the second part can be referred to as the “soak.”

Another alternative for normalizing includes charging the composition at an initial furnace temperature at about 1500° F., and maintaining the furnace temperature at about 1500° F. once the composition temperature approaches 1500° F. This alternative only uses the soak portion of the process. The process will work in such a manner, but the time must be increased accordingly.

In another alternative, a shorter or longer thermal head time may be utilized, with the time depending on the first temperature of the furnace. In summary, the process heats the bars above the transformation temperature (typically about 1450° F.), and keeps them at that higher temperature for some time.

Also, the normalizing temperature used depends on the specific chemistry, or combination of elements, of the steel, though temperatures in the range of about 1500° F. to about 1650° F. are expected. Depending on the composition, the initial furnace temperature and second furnace temperature will vary from the example discussed above. For instance, in another alternative, the first initial furnace temperature may be 1625° F. When the composition surface temperature approaches 1525° F., then the furnace temperature is reduced to 1525° F. to complete the normalizing. Following normalization, the product exits the furnace and is allowed to cool on an exit conveyor.

In one embodiment, a steel reinforcing bar may be created using a rolling process from the composition. The bar meets or exceeds the requirements of ASTM A615 Standard Specification for Deformed and Plain Billet-Steel Bars for Concrete Reinforcement, which are as follows:

Minimum Yield Strength (ASTM A370-03a): 75,000 psi;

Minimum Tensile Strength (ASTM A370-03a): 100,000 psi;

Minimum Elongation (ASTM A370-03a): 10%;

Bend Test 9d Pin (ASTM A370-03a): 90 degrees;

In addition, the bar exhibits a minimum Charpy V-Notch Impact Strength at −20° F. (ASTM A673) of at least 15 ft-lb.

The invention will now be described further by way of the following examples.

EXAMPLE 1

Heat S61270, with a grade description of 75S-M5, had a composition including iron and other untested elements as well as the following elements with their amounts: Element % C 0.32 Mn 1.43 P 0.02 S 0.018 Si 0.42 Sn 0.01 Cu 0.33 Ni 0.25 Cr 0.18 Mo 0.04 Cb 0.002 Al 0.001 N 0.02 Co 0.01 Ti 0.003 V 0.132 Ca 0.0007

This composition was formed into bars and then charged to a furnace with an atmospheric temperature of about 1600° F. The bars were allowed to heat until the surface of the bars approached about 1500° F. This heating required about 20 minutes. Then, the temperature of the furnace was reduced to about 1500° F. The bars were held at this temperature for about 35 minutes. Thereafter, the bars exited the furnace and were allowed to cool on the exit conveyor.

The composition was then tested. The yield strength of the composition was 81.7 k.p.s.i. and the tensile strength was 108.3 k.p.s.i.. Additionally, the composition has an elongation test result of 20.63% and the Charpy impact strength was 35.5 ft-lbs.

EXAMPLE 2

Heat S73516, with a grade description of 75S-M7, had a composition including iron and other untested elements as well as the following elements with their amounts: Element % C 0.32 Mn 1.40 P 0.012 S 0.009 Si 0.23 Sn 0.018 Cu 0.32 Ni 0.24 Cr 0.11 Mo 0.034 Cb 0.002 Al 0.002 N 0.0191 Co 0.011 Ti 0.003 V 0.144 Ca 0.0012

This composition was formed into bars and then charged to a furnace with an atmospheric temperature of about 1600° F. The bars were allowed to heat until the surface of the bars approached about 1500° F. This heating required about 20 minutes. Then, the temperature of the furnace was reduced to about 1500° F. The bars were held at this temperature for about 35 minutes. Thereafter, the bars exited the furnace and were allowed to cool on the exit conveyor.

The composition was then tested. The yield strength of the composition was 80.7 k.p.s.i. and the tensile strength was 105.5 k.p.s.i.. Additionally, the composition has an elongation test result of 18.8% (8 inch gage length) and the Charpy impact strength was 30.8 ft-lbs.

EXAMPLE 3

Heat S74110, with a grade description of 75S-M7, had a composition including iron and other untested elements as well as the following elements with their amounts: Element % C 0.31 Mn 1.41 P 0.012 S 0.012 Si 0.26 Sn 0.01 Cu 0.35 Ni 0.33 Cr 0.14 Mo 0.04 Cb 0.001 Al 0.001 N 0.0197 Co 0.01 Ti 0.003 V 0.149 Ca 0.0021

This composition was formed into bars and then charged to a furnace with an atmospheric temperature of about 1600° F. The bars were allowed to heat until the surface of the bars approached about 1500° F. This heating required about 20 minutes. Then, the temperature of the furnace was reduced to about 1500° F. The bars were held at this temperature for about 35 minutes. Thereafter, the bars exited the furnace and were allowed to cool on the exit conveyor.

The composition was then tested. The yield strength of the composition was 78.7 k.p.s.i. and the tensile strength was 107.8 k.p.s.i.. Additionally, the composition has an elongation test result of 20.6% (8 inch gage length) and the Charpy impact strength was 25.5 ft-lbs.

EXAMPLE 4

Heat S74248, with a grade description of 75S-M7, had a composition including iron and other untested elements as well as the following elements with their amounts: Element % C 0.30 Mn 1.41 P 0.011 S 0.012 Si 0.22 Sn 0.004 Cu 0.32 Ni 0.36 Cr 0.18 Mo 0.03 Cb 0.001 Al 0.002 N 0.0201 Co 0.01 Ti 0.002 V 0.141 Ca 0.0015

This composition was formed into bars and then charged to a furnace with an atmospheric temperature of about 1600° F. The bars were allowed to heat until the surface of the bars approached about 1500° F. This heating required about 20 minutes. Then, the temperature of the furnace was reduced to about 1500° F. The bars were held at this temperature for about 35 minutes. Thereafter, the bars exited the furnace and were allowed to cool on the exit conveyor.

The composition was then tested. The yield strength of the composition was 85.6 k.p.s.i. and the tensile strength was 111.4 k.p.s.i.. Additionally, the composition has an elongation test result of 17.8% (8 inch gage length) and the Charpy impact strength was 36.2 ft-lbs.

A number of embodiments of the invention have been described. Nevertheless, it will be understood the various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims. 

1. A steel composition, comprising: about 0.25-0.37% by weight Carbon; about 1.20-1.55% by weight Manganese; about 0.1-0.15% by weight Vanadium; about 0.20-0.40% by weight Nickel; about 0.20-0.50% by weight Silicon; about 0.30-0.45% by weight Copper; about 0.017-0.025% by weight Nitrogen; and Iron as the main constituent.
 2. The steel composition of claim 1, further comprising: from above zero up to about 0.30% by weight Chromium.
 3. The steel composition of claim 1, further comprising: from above zero up to about 0.035% by weight Phosphorus.
 4. The steel composition of claim 1, further compromising: from above zero up to about 0.04% by weight Sulfur.
 5. The steel composition of claim 1, further comprising: from above zero up to about 0.06% by weight Tin.
 6. The steel composition of claim 1, further comprising: from above zero up to about 0.06% by weight Molybdenum.
 7. The steel composition of claim 1, further comprising: from above zero up to about 0.30% by weight Chromium; from above zero up to about 0.035% by weight Phosphorus; from above zero up to about 0.04% by weight Sulfur; from above zero up to about 0.06% by weight Tin; and from above zero up to about 0.06% by weight Molybdenum.
 8. The steel composition of claim 1, further comprising: about 0.24-0.36% by weight Nickel; and about 0.22-0.42% by weight Silicon.
 9. The steel composition of claim 8, further comprising: from above zero up to about 0.25% by weight Chromium.
 10. The steel composition of claim 8, further comprising: from above zero up to about 0.25% by weight Chromium; from above zero up to about 0.025% by weight Phosphorus; from above zero up to about 0.04% by weight Sulfur; from above zero up to about 0.06% by weight Tin; and from above zero up to about 0.06% by weight Molybdenum.
 11. The steel composition of claim 1, further comprising: about 0.30-0.32% by weight Carbon; about 1.35-1.45% by weight Manganese; about 0.11-0.14% by weight by Vanadium; about 0.30-0.45% by weight Copper; about 0.019-0.021% by weight Nitrogen; and Iron as the main constituent.
 12. The steel composition of claim 11, further comprising: from above zero up to about 0.20% by weight Chromium.
 13. The steel composition of claim 11, further comprising: from above zero up to about 0.20% by weight Chromium; from above zero up to about 0.02% by weight Phosphorus; from above zero up to about 0.02% by weight Sulfur; from above zero up to about 0.06% by weight Tin; and from above zero up to about 0.04% by weight Molybdenum.
 14. The composition of claim 1, further comprising from above zero up to about 0.025% by weight Titanium; from above zero up to about 0.025% by weight Niobium; and from above zero up to about 0.04% by weight Aluminum.
 15. The composition of claim 1, further comprising from above zero up to about 0.025% by weight Titanium.
 16. The composition of claim 15, further comprising from above zero up to about 0.04% by weight Aluminum.
 17. The composition of claim 15, further comprising from above zero up to about 0.025% by weight Niobium.
 18. The composition of claim 1, further comprising from above zero up to about 0.025% by weight Niobium.
 19. The composition of claim 18, further comprising from above zero up to about 0.04% by weight Aluminum.
 20. The composition of claim 1, further comprising from above zero up to about 0.04% by weight Aluminum.
 21. A method of making a steel, comprising: (a) providing a steel composition, comprising: about 0.25-0.37% by weight Carbon; about 1.20-1.55% by weight Manganese; about 0.1-0.15% by weight Vanadium; about 0.20-0.40% by weight Nickel; about 0.20-0.50% by weight Silicon; about 0.30-0.45% by weight Copper; about 0.017-0.025% by weight Nitrogen; and Iron as the main constituent. (b) charging the composition into a furnace; and (c) normalizing the composition by heating at a furnace temperature between about 1500° F. and about 1650° F.
 22. The method of claim 21, wherein the steel comprises a 75S steel.
 23. The method of claim 21, further comprising: (d) tempering the composition by heating at a furnace temperature between about 850° F. and about 1000° F.
 24. The method of claim 21, wherein the normalizing step comprises: charging the composition at an initial furnace temperature at about 1600° F.; and lowering the furnace temperature to a furnace temperature at about 1500° F. once the composition temperature approaches 1500° F.
 25. The method of claim 24, wherein the composition is held at the initial furnace temperature for about 15 to 30 minutes, and wherein the composition is held at the second furnace temperature for about 30 to 45 minutes.
 26. The method of claim 21, wherein the steel composition provided is in the form of bars, ingots, plates, or sheets.
 27. The method of claim 21, wherein the normalizing step comprises: charging the composition at an initial furnace temperature at about 1500° F.; and maintaining the furnace temperature at about 1500° F. once the composition temperature approaches 1500° F.
 28. The method of claim 21, further comprising: (d) cooling the composition in air.
 29. A steel anchor bolt, comprising: about 0.25-0.37% by weight Carbon; about 1.20-1.55% by weight Manganese; about 0.1-0.15% by weight Vanadium; about 0.20-0.40% by weight Nickel; about 0.20-0.50% by weight Silicon; about 0.30-0.45% by weight Copper; about 0.017-0.025% by weight Nitrogen; and Iron as the main constituent.
 30. The steel anchor bolt of claim 29, further comprising: from above zero up to about 0.30% by weight Chromium; from above zero up to about 0.035% by weight Phosphorus; from above zero up to about 0.04% by weight Sulfur; from above zero up to about 0.06% by weight Tin; and from above zero up to about 0.06% by weight Molybdenum.
 31. The steel anchor bolt of claim 29, wherein said steel anchor bolt meets the requirements of 75S steel. 